Raman spectroscopy has established itself over the last few decades as a powerful analytical for determining the composition of a chemical sample. There are a wide range of materials that Raman spectroscopy can be used on, and in this article, we look at how Raman spectroscopy is applied to examining and identifying mineral samples.
What is Raman Spectroscopy?
Raman spectroscopy is a type of vibrational spectroscopy and is used to identify the different vibrational, rotational, and other low-frequency modes of a molecule. The identified modes are then used to determine the chemical structure of the molecule(s) in the sample, as well as the polymorph, stress on a molecule, a molecule’s orientation, and its crystallinity. It is a method that is widely used on inorganic materials and those which exhibit a solid-state lattice structure.
Raman spectroscopy utilizes the Raman effect, which is an effect that occurs when the electric dipole interacts with a photon of light. Raman spectroscopy relies on the inelastic scattering of light (Raman scattering). This means that when the incident photon of light is scattered by a molecule, the frequency of the scattered photon is different. When the photon interacts with the sample, the electric field of the molecule becomes perturbed and excited to a virtual energy state. When the molecule returns to its ground state, the molecule’s vibrational energy changes, and this change can then be detected by the spectrometer.
There are many different components that enable a Raman spectrometer to work. The incident photons are generated from a laser before passing through a lens and a monochromator. This process isolates a single wavelength of light that is shone on to the sample in question. A filter is then used to collect any wavelengths that are the same as the incident wavelength (i.e. elastically scattered wavelengths), and the inelastically scattered wavelengths are picked up by a charge-couple device (CCD) detector. This spectrum is then analyzed.
How Raman Can Identify and Examine Minerals
Raman spectroscopy has established itself as a method that can deduce a lot of information about many different materials, and this extends out to geological samples such as minerals. Minerals are very complex materials, but because their basic composition is inorganic and solid-state in nature, Raman has become a tool that can provide the necessary information on the composition a mineral sample—even for different structural groups and mineral phases within the same mineral class.
Raman spectroscopy is an excellent tool for differentiating between minerals based on their class and chemical constituents. Minerals can be analyzed in a number of forms, including in rocks and in soil samples, and it is a technique that can be used for identifying multiple minerals in a sample. For many years, X-ray diffraction (XRD) was the method of choice for analyzing mineral samples. However, XRD requires the sample to be ground into a powder if it is to be analyzed, but because Raman spectroscopy is a non-destructive method that can be used with a wide range of sample types, it has become the go-to method for many scientists.
One of the big reasons for using Raman spectroscopy in mineral analyses is that it can distinguish between carbonates and silicates. Both carbonate and silicate rocks are the most abundant rock types on Earth, but many other methods have trouble distinguishing between the two. For these minerals, Raman spectroscopy offers a way to identify between silicates and carbonates by identifying the [SiO4]4- tetrahedron ions and the CO33- ions, respectively. For silicates, it can also differentiate between isolated tetrahedrons and tetrahedra chains—as well as what positive ions fill the atomic gaps of these tetrahedrons—which enables minerals that have the same composition, but a different structural arrangement, to be identified. Likewise, for carbonate-based minerals, the associated positive ion can also be deduced and distinguished from other elements, which enables a more accurate identification of carbonate minerals to be possible.
Another reason why Raman spectroscopy has become an effective method for examining minerals, is because it can analyze fluids and gas bubbles that are trapped in the mineral. This is a common occurrence in mineralogical samples, but not many methods can analyze these trapped fluids and gas bubbles. Raman spectroscopy can also be used to image these fluids if a confocal microscope is attached to the Raman spectrometer and the correct objective is used.
Sources and Further Reading
- Horiba: http://www.horiba.com/fileadmin/uploads/Scientific/Documents/Raman/Geo01.pdf
- “Raman spectroscopy for mineral identification and quantification for in situ planetary surface analysis: A point count method”- Viskupic K. M. et al, Journal of Geophysical Research, 1997.
- “A powerful tool for material identification: Raman spectroscopy”- Vaskova H., International Journal of Mathematical Models and Methods in Applied Sciences, 2011.
- “Identification of cave minerals by Raman spectroscopy: new technology for non-destructive analysis”- International Journal of Speleology, 2006.